For many years it has been known that the stretching of an orientable thermoplastic material under certain conditions, now well known in the art, would result in the orientation, i.e. alignment, of the molecules of the material in the direction of stretching. One method whereby this stretching may be accomplished is known as the "bubble" process.
The bubble process is a well known process for forming an oriented thermoplastic film wherein an extruded tube of thermoplastic material which is heated to its orientation temperature range is sequentially inflated and stretched by internal pressure, cooled and then collapsed into a lay-flat configuration. The collapsed tube may subsequently be wound up in roll fashion for storage. The tube may be cut or slit to form a planer film. The tube is usually extruded vertically. After extrusion a volume of air is trapped within the tube . The internally trapped air causes the extruded tubing to assume a bubble or balloon-like configuration so as to enlarge, stretch and orient the tube in both the transverse and longitudinal directions. The bubble may be formed through utilization of two sets of pinch rolls which may also serve to collapse the tube and form a lay-flat film. The thickness of the film may, to some degree, be controlled by varying the volume of the internally trapped air and hence the degree of enlarging and stretching, by varying the rate of extrusion and/or varying the speed of revolution of the pinch rolls which collapse the tube into lay-flat configuration.
The terms "oriented" and/or "orientation" are used herein to describe the process and resultant product characteristics obtained by stretching a resinous orientable polymeric thermoplastic material which has been heated to within its orientation temperature range and then cooled in order to lock-in or "freeze" the molecular alignment of the material in the direction of stretching. This action improves the mechanical properties of the film, such as, for example, shrink tension and orientation release stress. Both of these properties may be measured in accordance with ASTM D 2838 69 (reapproved 1975). The orientation temperature range for a given film will vary with the different resinous thermoplastic materials or blends thereof which comprise the film. However, the orientation temperature range may generally be stated to be above room temperature and below the melting point of the thermoplastic material or blend of materials. Orientation temperature ranges for the materials encompassed by the present invention are well known to those skilled in the art. When the stretching force is applied in one direction uniaxial orientation results. When the stretching force is applied in two directions biaxial orientation results.
The term "cross-oriented" is used herein to describe a multilayer film comprising two or more layers in which at least two of the layers are oriented at an angle with respect to each other.
The terms "polymer" and "polymeric" are used herein to include polymers, ionomers, copolymers, interpolymers, homopolymers, block or graft polymers and blends thereof.
Other methods of stretching are known in the art. For example, it has been recognized in the art that the extruded tubing may be longitudinally stretched by revolving the pinch rolls which initially collapse the tubing after extrusion at a rate in excess of the linear velocity with which the tubing emerges from the extrusion die. If the temperature of the extruded tubing is maintained within its orientation temperature range during the stretching, the molecules of the tubing will be oriented in the direction of stretching. Films manufactured by this method are generally referred to as hot stretched. It has also been recognized in the art that the extruded tubing may be longitudinally stretched by revolving one of the pairs of pinch rolls which transport a tubular extrudate, which has been extruded, cooled and reheated to its orientation temperature range, at a rate in excess of the rate of revolution of a preceding pair of pinch rolls. Films manufactured by this method are generally referred to as cold stretched films. Either of these methods accomplishes some degree of orientation of the stretched tubular extruded film in the longitudinal or tubular direction. However, if a high degree of orientation is desired the later procedure should be followed since it results in a greater degree of orientation. Furthermore, it is also well known that the transverse expansion of an extruded tubular film, which is heated to a temperature within its orientation temperature range results in the stretching and consequent orientation of the tubular extruded film in the transverse or lateral direction. A greater degree of transverse orientation occurs if the extruded material is first cooled and then reheated to its orientation temperature range (i.e. cold stretched) prior to being subjected to transverse stretching and expansion. If the transverse stretching is coupled with longitudinal stretching, as is the case in the bubble process, a biaxial orientation is imparted to the resultant extruded film.
Stretching to orient a thermoplastic material is widely utilized within the art since it is well known that an oriented material exhibits increased tear resistance in the direction transverse to the direction of stretching and orientation. Further discussion of film orientation may be found at Volume I, Chapter 10 of The Science and Technology of Films, copyrighted in 1968 by John Wiley and Sons. The book was edited by Orville J. Sweeting and is hereby incorporated by reference.
It is also previously known to prepare a multi-layer plastic film by means of two or more extruders, a die with several concentric annular extrusion slots, bubble stretching apparatus and/or longitudinal stretching or drawing apparatus. The materials which form the layers of said plastic film are separately prepared and processed in the different extruders and then are passed separately to the different concentric extrusion slots of the die. The diameters of the concentric die slots are very close to each other. Thus, the emerging tubes which form the different layers of the final film make contact with each other shortly before or after emerging from the die and are stretched and oriented only after being united with each other. Such action means that the individual layers cannot be independently stretched and oriented. Thus all layers of the extruded film must have the same direction of orientation.
The present invention relates to a new continuous process by means of which it is possible to prepare a multi-layer film having a different orientation distribution between the film layers. In a preferred embodiment, the present invention comprises a film whereby an interior layer of the extruded tubular structure is predominantly oriented in the longitudinal direction and an exterior layer of the extruded tubular structure is predominately transversely oriented. The thus formed cross-oriented tubing has improved strength and high tear resistance.
Other individuals have worked in this area and, indeed, have received patents on cross-oriented films and methods for producing them. For example, the work of Reifenhauser et al. has resulted in U.S. Pat. Nos. 3,726,743 and 3,926,706. Additionally, the work of Kubat et al. resulted in U.S. Pat. No. 4,076,568. While these patents do disclose methods for producing cross-oriented films and cross-oriented film products, the apparatus, processes and products disclosed in these patents are distinguishable from those of the present invention in that the above-identified patents generally disclose much more complex apparatus whereby the tubular layers are independently extruded or coextruded and the various cross-orientations are regulated by varying the pressure of the air injected into the inner tube and/or injected between the inner and outer tubes, or both. Thus, it can be seen, prior to the present invention, the need existed for a much more simple and more readily controllable process to produce oriented multi-layer films having a different orientation distribution between the layers of the film.